1. Field of Invention
This invention relates to soft and flexible electrical heaters, and particularly to heating elements, which have soft and strong metal or carbon containing electrically conductive textile threads/fibers.
2. Description of the Prior Art
Heating elements have extremely wide applications in consumer household products and in, construction, industrial application, etc. Their physical characteristics, such as thickness, shape, size, strength, flexibility and other characteristics affect their usability in various applications. Numerous types of thin and flexible heating elements have been proposed. For example, a heating element proposed by Ohgushi (U.S. Pat. No. 4,983,814) is based on a proprietary electro conductive fibrous heating element produced by coating an electrically nonconductive core fiber with electro conductive polyurethane resin containing the carbonatious particles dispersed therein. Ohgushi""s manufacturing process appears to be complex; it utilizes solvents, cyanides and other toxic substances. The resulting heating element has a temperature limit of 100xc2x0 C. and results in a pliable but not soft heating element. In addition, polyurethane, used in Ohgushi""s invention, when heated to high temperature, will decompose, releasing very toxic substances, such as products of isocyanides. As a consequence, such heating element must be hermetically sealed in order to prevent human exposure to toxic off gassing. Ohgushi claims temperature self-limiting quality for his invention; however xe2x80x9cactivationxe2x80x9d of this feature results in the destruction of the heater. He proposes the use of the low melting point non-conductive polymer core for his conductive fabric-heating element, which should melt prior to melting of the conductive layer, which uses the polyurethane binder with the melting point of 100xc2x0 C. Thus, the heating element of Ohgushi""s invention operates as a Thermal Cut Off (TCO) unit, having low temperature of self-destruction, which limits its application.
U.S. Pat. No. 5,861,610 to John Weiss describes a heating wire, which is formed with a first conductor for heat generation and a second conductor for sensing. The first and second conductors are wound separately as coaxial spirals with an insulation material electrically isolating the two conductors. The two spirals are counter-wound with respect to one another to insure that the second turns cross, albeit on separate planes, several times per inch. The described construction results in a temperature sensing system, which can detect only the average change of resistance in the sensing wire due to elevation of the temperature in the heated product. Therefore, in the event of overheating of a very small surface area of the blanket or pad (for example, several square inches), the sensor may fail to detect a minor change of electrical resistance (due to operating resistance tolerance) along the heating element. In addition, such heating cable does not have inherent Thermal-Cut-Off (TCO) capabilities in the event of malfunction of the controller. The absence of the localized hot spot detection and the use of breakable metal wires make this heating element vulnerable to failure and not sufficiently safe for foldable products, such as heating pads and heating blankets.
Thrash (U.S. Pat. No. 5,801,914) describes an electrical safety circuit that utilizes two parallel conductors connected to a positive temperature coefficient material (PTC) and sacrificial fuse filament. Such sacrificial filament is connected to a separate switching circuit, which terminates electrical continuity of the PTC heating element in the event of fire hazard. The main disadvantages of this design are that (a) the switching circuit deactivates power only after arcing/fire has already started and burned the sensor fiber filament, thus producing a fire hazard to a heating product; and (b) the addition of a sensing sacrificial filament enlarges the overall thickness of conventional PTC cables, which already feature stiffness and bulkiness.
Gerrard (U.S. Pat. No. 6,310,332) describes an elongated heating element for an electric blanket comprising a first conductor means to provide heat for the blanket and extending the length of the element, a second conductor means extending the length of the element, and a meltdown layer between the first and second conductor means which is selected, designed and constructed or otherwise formed so as to display a negative temperature coefficient (NTC), and including electronic controller set to detect a change in the resistance of the meltdown layer to provide a means of changing the power supply to the first conductor means (providing heat to the blanket), to prevent destruction of the melt down layer. The element further includes a meltdown detection circuit for detecting meltdown of the meltdown layer and for terminating power to the first conductor means in the event that the control means fails and the meltdown layer heats up to a predetermined degree. The disadvantage of this construction is that the final safety of the blanket relies on a complex NTC/meltdown detection system located in the controller. In the event the controller fails, or significantly delays detection of NTC layer meltdown, then a severe scorching of the heating product, or fire hazard, can occur.
In the event a blanket user bypasses the controller by energizing the blanket directly from the power outlet, the heating element will not provide any overheat or fire hazard protection because the Gerrard""s heating element does not have inherent Thermal-Cut-Off (TCO) properties. The heating element utilizes winding of breakable metal wires, which makes construction thicker and more obtrusive for flexible heating products, such as heating pads and blankets.
Another disadvantage of the Gerrard""s invention is that its control system utilizes a half-wave power cycle for heating and another half-wave power cycle for meltdown stroke detection in order to provide proper heating output and meltdown protection. Therefore, the heating wire has to be twice thicker than comparable systems utilizing a full-wave power output. This feature becomes especially challenging for 120V and other lower voltage heating systems, compared to traditional European 240V systems. An increase in the thickness of heating wire leads to: (a) increase in the cost of heating conductor; (b) increase in the overall size of the heating element and (b) possibility of breaking the heating wires due to their reduced flexibility.
The present invention seeks to overcome the drawbacks of the prior art and describes the fabrication of a heater comprising metal fibers, metal wires, metal coated, carbon containing or carbon coated threads/fibers, which is economical to manufacture, does not pose environmental hazards, results in a soft, flexible, strong, thin, and light heating element core, suitable for even small and complex assemblies, such as hand wear. Significant advantages of the proposed invention are that it (a) provides for fabrication of heaters of various shapes and sizes with predetermined electrical characteristics; (b) allows for a durable heater, resistant to kinks and abrasion, and (c) with its electro-physical properties it is almost unaffected by abuses such as pressure, severe folding, small perforations, punctures and crushing. A preferred embodiment of the invention consists of utilizing electrically conductive textile threads/fibers having an inherent Thermal Cut Off (TCO) function to prevent overheating and/or fire hazard. The preferred system utilizes a NTC sensing layer for hot spot detection, which does not require having low-temperature meltdown characteristics. Because the proposed conductive fibers are extremely flexible, the coaxial winding process is not required in the heating element manufacturing, which makes the heaters extremely thin, light and durable. The heaters described in this invention may also comprise a continuous temperature PTC sensor to precisely control heating power output in the heating product. The control system may utilize the most economical full-wave power to vary heating output and to provide local hot spot detection.
The first objective of the invention is to provide a significantly safe and reliable heater which can function properly after it has been subjected to severe folding, kinks, small perforations, punctures or crushing, thereby solving problems associated with conventional flexible metal wire heaters. In order to achieve the first objective, the heater of the present invention may comprise (a) electrically conductive threads/fibers and (b) multi-layer insulation of the conductive threads/fibers. The conductive threads/fibers may be comprised of carbon, metal fibers, and/or textile threads coated with one or combination of the following materials: metal, carbon and/or electrically conductive ink. The proposed heater may also comprise metal wires and their alloys. The electrically conductive textile threads/fibers may possess the following characteristics: (i) high strength; (ii) high strength-to-weight ratio; (iii) softness and flexibility. The heating element core described in this invention is comprised of electrically conductive tapes, sleeves/tubes, sheets or cables, which radiate a controlled heat over the entire heating core surface. The multi-layer insulation of the electrically conductive threads/fibers provides increased dielectric properties, preventing or minimizing current leakage in the event of abuse of the heater. The multi-layer insulation may be applied in the form of encapsulation (through extrusion process) or lamination with insulating synthetic materials, having similar or different thermal characteristics.
A second objective of the invention is to provide maximum flexibility and softness of the heating element. In order to achieve the second objective, the electric heating element of the invention may contain thin (0.01 to 3.0 mm, but preferably within the range of 0.05-1.0 mm) conductive threads/fibers, which are woven, non-woven, knitted or stranded into continuous or electrically connected tapes, sleeves/tubes, cables or sheets. Another preferable configuration may consist of extruding soft insulating material, such as, but not limited to polyvinyl chloride (PVC), polyurethane, nylon, polypropylene, temperature resistant rubber, cross-linked PVC or polyethylene around a multitude of electrically conductive textile thread/fibers.
A third objective of the invention is to provide for the uniform distribution of heat, without overheating and hot spots, thereby preventing excessive insulation and improving energy efficiency. In order to achieve this objective: (a) conductive threads in the heating elements may be separated by non-conductive fibers/yarns or insulating polymers, (b) one side of the heating element may include a metallic foil or a metallized material to provide uniform heat distribution and heat reflection. It is also preferable that the soft heating elements of the invention are made without thick cushioning insulation, which slows down the heat delivery to the surface of the heating unit.
A forth objective of the invention is to provide a high level of temperature control. In order to achieve the forth objective, at least one metal wire and/or electrically conductive textile fiber runs throughout the heater, acting as a continuous temperature sensor. It is connected to an electronic power control regulator, which establishes a maximum power output limit for the heating product. It is preferable that such temperature sensor possess high positive temperature coefficient properties.
A fifth objective of the invention is to provide a high level of safety, minimizing the possibility of fire hazard. In order to achieve the fifth objective: (a) multiple thin heating cables may be reinforced by strong and flame retardant threads/fibers, (b) a negative temperature coefficient (NTC) sensor layer is applied to detect local overheating through the entire length of the heating element, (C) Positive Temperature Coefficient (PTC) or NTC continuous sensors may be applied to provide precise temperature control of the heating system, and (D) the conductive heating media of the heating cables may comprise metal or carbon containing electrically conductive textile threads/fibers with a polymer base having a melting temperature from 110xc2x0 C. to 350xc2x0 C. The melting of the conductive threads/fibers causes termination of the electrical continuity in the heating system. Thus, the proposed heating cables can operate as an inherent melting fuse or TCO (Thermal-Cut-Off) device.
The present invention comprises a heating element containing soft, strong and light electrically conductive textile threads/fibers acting as a heating means. The heating element is highly resistant to punctures, cuts, small perforations, severe folding and crushing. It can be manufactured in various shapes and sizes, such as cables, strips fabrics or sleeves, and it can be designed for a wide range of parameters, including but not limited to input voltage, temperature, power density, type of current (AC or DC) and method of electrical connection (parallel or in series). The heating element may contain non-conductive fibers/yarns or insulating polymers which are combined with electrically conductive individually insulated metal or carbon containing threads/fibers by knitting, weaving into or, laminating between layers of woven or non-woven fabric or sheeting, forming tapes, sleeves/tubes or sheets.
Selected areas of the heating element may contain electrically conductive textile fibers or wires to provide continuous PTC temperature sensing and/or may act as regular electrical conductors (collectively: xe2x80x9cheat detection meansxe2x80x9d) to provide an electrical signal to the electronic controller. The NTC sensing layer is located between such heat detection means and the heating electrically conductive textile threads/fibers (xe2x80x9cheating meansxe2x80x9d). The electrically conductive textile fibers also act as a continuous thermal fuse, terminating continuity in the heater at the temperatures 110xc2x0 C.-350xc2x0 C. as dictated by the heating element design.
The heating element may be shaped by folding, turning, molding, weaving, stitching, fusing, and/or laminating or by any other appropriate assembling technique to obtain the predetermined configuration of the heater. The electrical terminals, such as connector pins, crimps or electrodes may be attached to the ends of said heating element. The electrically conductive textile fibers may be electrically connected in parallel or in series.